Nitric oxide (NO) is a small, highly reactive gas synthesized by the enzyme NO synthase (NOS). NOS activity in murine leukocytes has been shown to play an important role in the murine immune response. However, the role of NOS in the human immune system remains highly controversial because NOS-associated effects in human leukocytes have been very difficult to demonstrate. Our preliminary data indicate that very low levels of NOS expressed constitutively in human lymphocyte or monocyte-derived cells lines regulate apoptosis. We hypothesize that NOS activity regulates apoptosis in human primary lymphocytes and/or monocytes as well as in cell lines, and therefore, that NOS activity has a physiologically relevant role in the human immune system. In Specific Aim 1, we will test this hypothesis by determining if NOS-activity in human primary lymphocytes and monocytes regulates spontaneous or Fas-induce apoptosis. In Specific Aim 2, human lymphocytic cell lines will be used as models to investigate the mechanism by which NOS inhibits Fas-induced apoptosis. Our hypothesis is that NOS activity inhibits Fas-induced apoptosis at the level of Fas-associated protein complex formation. The proposed studies will contribute ot the immunology and nitric oxide fields. The studies will clarify whether NOS activity has a physiologic role in the human as well as the murine immune system: will provide insight into the mechanism by which NOS activity exerts biological effects in human cells; and will elucidate a novel NOS-dependent regulation of Fas-induced apoptosis. Ultimately these studies may lead to the development of NO-based therapies to treat disorders associated with dysregulation of apoptosis including autoimmune diseases, cancer and the acquired immunodeficiency syndrome.